Recently, optical data recording media are required more and more to be enhanced with their data recording density in order for the increased data quantity such as video pictures to be processed. One of solution approaches thereto is a super-resolution technique. The super-resolution technique is a technique for reproducing data from marks (marks herein are intended to mean pre-pits as well as recording marks generated when data is recorded to a recording type optical data recording medium) which have a length equal to or shorter than the optical resolution limit involved in a reproducing device (the limit is determined in accordance with the laser wavelength and numerical aperture of the optical system), and this technique thus allows for recording with short length marks thereby to substantially increase the data recording density.
To this end, various optical data recording media have been heretofore proposed to reproduce data from shorter length marks than the optical system resolution limit involved in a reproducing device (such optical data recording media will be referred to as “super-resolution media (medium)” hereinafter). Patent Document 1 and Patent Document 2 describe, as one or more examples, approaches for substantially reducing the spot diameter by forming a mask layer in an optical data recording medium.
Specifically, Patent Document 1 discloses an optical disc device for performing recording and reproducing through using reflectivity changes caused by pre-pits having convexoconcaves formed on the substrate of an optical disc medium, wherein a temperature dependent optical shatter layer is formed on a medium layer recorded therein with signals (the side of disc surfaces to which laser beam is to be irradiated from the objective lens) such that the shatter layer absorbs the reproduction laser beam at room temperature but is disabled to absorb the reproduction laser beam due to the increase in temperature caused by the reproduction laser power while on the other hand absorbing the reproduction laser beam due to the decrease in temperature after passing reproducing focus spots.
Patent Document 1 also discloses an optical disc device for performing recording and reproducing of data through using reflectivity changes caused by phase transition in a recording material, wherein a temperature dependent optical shatter layer is formed on a medium layer recorded therein with data (the side of disc surfaces to which laser beam is to be irradiated from the objective lens) such that the shatter layer absorbs the reproduction laser beam at room temperature but is disabled to absorb the reproduction laser beam due to the increase in temperature caused by the reproduction laser power while on the other hand absorbing the reproduction laser beam due to the decrease in temperature after passing reproducing focus spots.
On the other hand, Patent Document 2 discloses an optical data recording medium having a recording film on a substrate, wherein the recording film is provided therein with an optical shatter layer which is disposed at the front-end location in terms of incident reading light or recording light and which contains a thermochromism substance as a main component, which is in a chromogenic status not transmitting light before the reading light is irradiated while the center area of the irradiated region is partially extinguished in color to be optically transparent in accordance with an increase in temperature due to the irradiation of reading light.
These approaches involve mask layers (optical shatter layers) formed of substances, such as thermochromism material and phase transition material, which are to melt when the temperature is increased up to a certain value or higher, thereby exhibiting mask effects. A substance in molten status has a high fluidity, so that the composition and shape in the initial status are easily to change. Consequently, in an optical data recording medium having a mask layer capable of exhibiting the mask effect when the temperature increases up to a certain temperature or higher, if repeating the recording and/or the reproducing, then a problem occurs that the mask effect is steadily reduced due to the shift in composition or shape of the mask layer and substantially lost after repeating around several thousand times. The above conventional optical data recording medium is thus insufficient in durability.
As a method for solving this problem, Patent Document 3 and Patent Document 4 disclose an invention using as a mask layer a metal oxide which changes its reflectivity and/or transmissivity in response to the change in temperature. Patent Document 3 and Patent Document 4 also describe that the mask effect is prevented from deterioration even if repeating the recording and/or the reproducing, by forming the mask layer of a substance, such as a metal oxide (preferably zinc oxide), which does not melt due to the increase in temperature during the recording and/or the reproducing.
Further, Patent Document 5 describes that a layer to contact the data recording layer as the mask layer is preferred to have a low thermal conductivity in order to accelerate the temperature rising and that a transparent resin layer is preferred to be interposed between a light permeable base material film and an data recording layer to combine them.
Incidentally, the optical data recording medium as described in Patent Document 5 is configured such that the light permeable base material film, the transparent resin layer, the data recording layer, and a substrate are laminated, and the mask layer forming the data recording layer is continuously to contact the transparent resin layer. Accordingly, the functionality of the transparent resin layer is required to involve an embedding capability (following capability) to the data recording layer having convexoconcaves while sufficiently maintaining an adhesion with a metal oxide forming the data recording layer and further not to negatively affect the metal oxide.
In general, as a means for improving the adhesion with metal, a method is employed which makes an acidic group be contained in the component in the transparent resin layer. In this case, however, corrosion may be caused in the metal oxide film contacting the transparent resin layer particularly under high temperature and high humidity. If, on the other hand, an acidic group is incorporated, then corrosion may be suppressed, but, conversely, the adherence to metal is drastically reduced.
As a method to solve this problem, Patent Document 6 proposes to use a pressure-sensitive adhesive agent which contains no carboxyl group-containing co-polymerizable monomer but contains a nitrogen atom-containing co-polymerizable monomer. However, with respect to (meth)acrylate copolymer synthesized according to Patent Document 6, the behavior of the properties of the transparent resin layer is not mentioned at all, and if employing the configuration as mentioned in embodiments thereof, then cracks may occur during rapid environmental changes in the film of metal oxide which forms the data recording layer.